Our last class this term, which I nicknamed FUN FRIDAY. We made fizzing bath bombs, butter and ice cream.
The younger group started out with an experiment you find all over the internet, often called “The Cat’s Meow”. It is one of the experiments in the series I linked to at the start of the term. My youngest son had tried it and wanted to show it to his friends. We did two different versions – one with detergent and one with oil.
We set out two plates of full-fat milk, and dropped tiny amounts of food colouring in positions evenly spaced around the plates. The food colouring (which is water soluble) did not spread out very much.
We then added detergent to the centre of one plate, and oil to the centre of the other.
The oil did not do much, although if I stood with my head at the right angle, I could see a thin-film interference pattern (not something that I could easily capture on my smartphone). I added the oil as I had seen it mentioned on the internet and wanted to see if it would work.
When we added detergent to the other plate, the coloured spots started to swirl around. That’s what I was looking out for!
I believe the effect we see here is not completely understood, but is undoubtedly to do with the nature of detergent. Each detergent particle has one end that bonds with water (the hydrophilic end) and one end that bonds with fat (hydrophobic). When we wash dishes or clothes, the detergent particles surround the oil or fat globules so they can be carried away in the water. Perhaps the same process in milk sets up currents which are made visible by the food colouring.
To explore this with the children, we poured oil and water into a glass. They all predicted correctly that the oil would float on the top of the water. I talked a little about how milk has microscopic fat globules inside it. I am not sure that they wanted to pursue the explanation very much more, and so we moved on to the next activity.
Here I set up some white powders and some colourless liquids that we had encountered before: sodium bicarbonate, cream of tartar and baking powder, plus demineralised water and ‘white vinegar’. We are all familiar with the reaction of bicarb with vinegar. This time I allowed the children to set up their own ‘experiments’: each child had two containers and could choose what to put in them. They wrote down what was in each container and we briefly discussed the result (trying to clear up at the same time). I have not asked them to be quantitative yet, but I am hoping that this will make it clear that not all white powders react with vinegar to give effervescence. For example, my son tried tartaric acid and vinegar and nothing happened. I briefly recapped that the vinegar + bicarb reaction is an example of ACID plus ALKALI. Maybe in another term I can set them an analytical chemistry activity where they have to work out what a few ‘white powders’ are, by trying out different reactions. We can also use red cabbage water as a home-made indicator solution.
Finally, we came to making our bath bombs.
I had prepared a mixture of bicarb and citric acid the previous night, and despite keeping it in a screw-top container, it had obviously absorbed water from the atmosphere and was pretty rock hard in the bottom of the container. We had to jab at it with a knife but managed to dislodge the whole mixture into a mixing bowl where we added oil. Each child wore gloves and had a go at moulding the mixture. I had neglected to supply safety glasses and one child got citric acid in their eye but a parent used plenty of water to wash it out. I will definitely supply safety glasses next time.
Bath bombs, if you are not familiar with them, are yet another example of the ACID + ALKALI (or ACID + BASE) reaction. And if we use sodium bicarbonate as the base, it reacts to give a salt, water and carbon dioxide. So, drop your bath bombs into some warm water and you have a fizzy bath. We did not add any food colourings, or essential oils for fragrance, but they are easily included in the recipe.
I probably should have checked beforehand whether the children all had baths at home. At least one family only has showers, so will try the bath bombs in the wash basin. But it should be fun anyway.
I used instructions I wrote myself ages ago when I was working for Planet Science in the UK. I see my version has disappeared from their site (I said it was ages ago, didn’t I?) but they have new instructions here.
I went over time (again) and started the older group late, but they did not mind as we got to make ice-cream with no freezer, and eat it too:
We froze our ice cream by nestling our small containers of cream and sugar in a bowl of ice cubes. We added salt to the ice to bring its temperature down – ask me if you want an explanation of this one.
(The girls aren’t officially in the older group but scored some ice cream simply by being in the right place at the right time.)
We also made our own butter:
Again, you can find instructions on Planet Science, although they are not specific enough for me. I reckon it takes between 10 and 15 minutes to make the butter, and it can get very tiring while you are shaking. Perhaps the new writers didn’t want to put off their readers!
In both of these activities, we were using the properties of milk and the properties of salt, and looking at physical changes rather than chemical changes.
I have to move house now. I will not be running classes for about six weeks, and may have sporadic internet access. I will try to post articles and interesting links when I can. Next term’s classes start on 15 August, and will cover The Physical World: heat, light, sound and forces.